The present invention relates to electrical test apparatus and, more particularly, to apparatus for identifying and testing individual electrical conductors in a bundle of unidentified conductors.
Apparatus for identifying electrical connectors within a group of electrical connectors either intentionally or unintentionally (shorted) electrically interconnected at a remote location is not new. For example the simple "bell and battery" of FIG. 7 is well known and well used in the electrical arts. Given a pair of cables 150 and 152 comprising a plurality of insulated electrical conductors 154 and 156, respectively, any electrical interconnection between a conductor 154 and a conductor 156 can be determined by sequentially applying the test leads 158 of the test set, generally indicated as 160, to the possible combinations of conductors 154 and 156. As can be seen in the circuit diagram of FIG. 8, the test set 160 comprises a battery 162 connected in series with a bell 164 between the two test leads 158. When connected to an electrically interconnected pair of conductors 154, 156 as shown in FIG. 8, the circuit is completed between the test leads 158 and the bell rings.
Telephone systems present unique problems in the testing and identification of electrical cables. Telephone networks employ multi-conductor cables to interconnect remotely located telephone switching systems (such as that located at the central office) to other switching systems or subscriber equipment. The multi-conductor cables comprise a plurality of twisted wire pairs. A "pair" comprises the two wires that are used to connect the central office and subscriber equipment. One wire of a pair is referred to as the "tip" and the other as the "ring". Each pair of wires is bundled in groups of 25 or 100 pairs. Cables, in turn, may include as many as 3600 pairs. Cable is placed, whether aerial or underground, in sections. A typical 1200 pair cable reel length of 22 AWG gauge wire, with polyethylene conductor insulation, is 1250 feet in length. Splices are required throughout the cable network to connect such sections to one another and also to connect main cables with feeder and distribution cables of smaller cable pair count.
Until recently, the splicing (wire-joining) method commonly employed in the telephone industry involved splicing each individual pair by joining the tip wires, one to another, with discrete connectors, and the ring wires, one to another, each with a second discrete connector. Such a process is both time-consuming and expensive.
Recently, a method and associated hardware has been adopted in the telephone industry to perform the splicing operation through the joining of groups of pairs (in many cables, wire pairs are bundled in 25-pair groups). This is commonly referred to as "modular splicing".
Modular splicing equipment includes what is commonly referred to as a "cutter-presser" device in which a plastic module, comprising several parts, is employed. Examples of such splice modules are ones manufactured by the 3M Company, St. Paul, Minn. and sold under the name MS.sup.2 module and one called the 710 Connector used in the Bell System (described in Bell System practice section 632-205-222, Issue 1, October, 1973). Individual wires are placed in identified slots in the module. When all of the wires are properly positioned in the module, the parts of the module are clamped to simultaneously provide electrical connection between appropriate wires and cut off the excess wire ends.
The presser device used in connection with the 710 Connector also includes means for providing electrical access to the wires in the module through test ports in the module body. An electrical connector on the cutter-presser equipment provides access to the 25 pairs spliced into the module. Various types of test sets could, therefore, be connected to the cutter-presser device for testing using the test ports and the electrical connector provided. Functionally similar access to the MS.sup.2 module is also provided for connecting test equipment. While provision has thus been made for test equipment, to date, no test equipment is available for interfacing through the provisions thus provided to allow rapid and accurate testing of the type of cable splice which normally occurs in cable rearrangements using such apparatus.
There are many reasons for rearranging cables. For example, a section of cable may become faulty and need to be replaced. A cable route may have to be relocated due to a change in surface or underground conditions. Increased facilities over a particular cable route may be required from a certain point in the field to a more distant subscriber terminal equipment point. In many if not most, cable transfers, telephone operating companies attempt to make such transfer without interrupting service to the customer and often even perform the transfer while a voice conversation is being carried on the pair being physically respliced. To accomplish this without disruption or inconvenience to the customer imposes stringent limitations on what test apparatus can do in accomplishing its functions.
One type of transfer in which the cable test set of the present invention may be employed is shown as part of FIG. 1. In FIG. 1, there is shown an old cable (cable A) in which a section is to be replaced by a section of new cable (cable B). The new cable is spliced to the old cable at a first splice location using a bridge-tap or half-tap method, i.e. each wire in the old cable is tapped and a wire from a new cable is electrically connected in a "T" configuration. This bridge-tap or half-tap at the first splice location will not normally disturb a working line, even if in use, in normal voice communications. However, the critical phase of the transfer, which is normally referred to as "cut-closed" transfer, is where the free end of the new cable is now joined to the old cable at the second splice location. Unless the pairs are properly identified, and within each pair the proper polarity (ring-to-ring and tip-to-tip) are spliced, service will be interrupted. It is at this second splice location and for this identification and verification that the test apparatus of the present invention is to be used.
Additionally, the requirement for the inclusion of a battery, such as that employed in the "bell and battery" test set of FIGS. 7 and 8 is one which causes concern to users of such apparatus. Batteries are, typically, heavy and prone to give out at the moment of least convenience. Inasmuch as much of the previously discussed splicing and attendant testing is accomplished in locations which cannot easily be referred to as "convenient" (such as on raised poles and underground cable vaults), the elimination of a battery or other internal power supply for operation is a high priority design criteria.
Wherefore, it is the object of the present invention to provide a test set for accomplishing telephone cable splice testing and verification with apparatus requiring no internal power supply.